When spraying operations started in the morning, winds were very light from the southwest. Because the winds were so light, it was considered safe and more practical to land from the south and take off to the north rather than land and take off into wind and have to execute a hovering turn to position the helicopter for loading. Spraying operations are typically carried out in either no wind or light wind conditions, Pilots can become complacent about light winds, and may overlook subtle changes in wind speed or direction. At the time of the accident, winds had increased to only about six kph, with gusts. However, the helicopter was operating in a high-power, low-airspeed environment with a left, quartering tailwind, conditions conducive to unanticipated right yaw and loss of tail rotor effectiveness (LTE). When the uncommanded right yaw occurred, the helicopter veered toward a grove of trees. The pilot had no option but to pull in collective pitch to avoid striking the trees. The increase of collective control exacerbated the situation, and the aircraft began to rotate. Had the pilot's attempt to dump the spray load been successful, it is likely that he would have been able to quickly regain control of the helicopter and avoid the tree strike. The position of the Ag Nav remote switch, between the cyclic stick grip and the spray control box, made it difficult for the pilot to activate the dump switch because it was out of reach while his hand was on the cyclic. While he was attempting to dump the spray load, the pilot's middle finger was likely just catching the edge of the dump switch and sliding off the switch rather than fully depressing it.Analysis When spraying operations started in the morning, winds were very light from the southwest. Because the winds were so light, it was considered safe and more practical to land from the south and take off to the north rather than land and take off into wind and have to execute a hovering turn to position the helicopter for loading. Spraying operations are typically carried out in either no wind or light wind conditions, Pilots can become complacent about light winds, and may overlook subtle changes in wind speed or direction. At the time of the accident, winds had increased to only about six kph, with gusts. However, the helicopter was operating in a high-power, low-airspeed environment with a left, quartering tailwind, conditions conducive to unanticipated right yaw and loss of tail rotor effectiveness (LTE). When the uncommanded right yaw occurred, the helicopter veered toward a grove of trees. The pilot had no option but to pull in collective pitch to avoid striking the trees. The increase of collective control exacerbated the situation, and the aircraft began to rotate. Had the pilot's attempt to dump the spray load been successful, it is likely that he would have been able to quickly regain control of the helicopter and avoid the tree strike. The position of the Ag Nav remote switch, between the cyclic stick grip and the spray control box, made it difficult for the pilot to activate the dump switch because it was out of reach while his hand was on the cyclic. While he was attempting to dump the spray load, the pilot's middle finger was likely just catching the edge of the dump switch and sliding off the switch rather than fully depressing it. The helicopter took off downwind, conducive to unanticipated right yaw; the helicopter yawed to the extent that control was lost and could not be recovered before it struck a tree. The Ag Nav remote switch, positioned between the cyclic stick grip and the spray control box, likely prevented the pilot from easily reaching the dump switch, adversely affecting a quick recovery of the helicopter.Findings as to Causes and Contributing Factors The helicopter took off downwind, conducive to unanticipated right yaw; the helicopter yawed to the extent that control was lost and could not be recovered before it struck a tree. The Ag Nav remote switch, positioned between the cyclic stick grip and the spray control box, likely prevented the pilot from easily reaching the dump switch, adversely affecting a quick recovery of the helicopter.